In recent years, chemiluminescence has been noticed as a detection system with high sensitivity in enzyme immunoassays. Meanwhile, a luminescence quantum yield in biological luminescence such as a luciferin-luciferase reaction is definitely higher than that in a chemiluminescence reaction, and the biological luminescence is thought to be suitable for microanalysis in the enzyme immunoassays. In particular, the quantum yield in the luminescence system in Cypridina (Cypridina noctiluca) is high and a turnover number of this enzyme is 10 times or more compared with firefly luciferase. Thus, the practical application of Cypridina luciferase to the enzyme immunoassays has been studied. Meanwhile, a method for preparing recombinant Cypridina luciferase in a large amount and a method for labeling Cypridina luciferase with biotin have been already disclosed in Patent document 1. However, in Patent documents 1 to 4 describing Cypridina luciferase, no example of the enzyme immunoassays using the biotin-labeled Cypridina luciferase was found, and the biotin-labeled Cypridina luciferase made according to the method described in Patent document 1 had only about 1.6% activity compared with the activity before modification. From these, it is obvious that the method for labeling Cypridina luciferase with biotin disclosed in Patent document 1 is not suitable for making the labeled enzyme applicable to the enzyme immunoassay. Meanwhile, in Patent documents 1 to 3, the method for maleimidizing the Cypridina luciferase is disclosed, but the activity of the Cypridina luciferase made by a maleimide hinge method was reduced to one tenth or less compared with the activity before the modification (described in Conventional Art in Patent document 4), and no amplification of signals by an avidin-biotin complex (ABC method) was obtained. Meanwhile, the enzyme immunoassay of the Cypridina luciferase utilizing a monoclonal antibody which recognizes Cypridina luciferase disclosed in Patent document 4 requires its high cost, and its practical application is not accomplished. Therefore, the development of a method for modifying Cypridina luciferase, which is suitable for the enzyme immunoassay, inexpensive and effective is desired.    Patent document 1: JP Hei-5-64583-A    Patent document 2: JP Hei-5-113443-A    Patent document 3: JP Hei-7-98316-A    Patent document 4: JP Hei-8-262021-A